1. Field of the Invention
The present invention relates to a vibration gyro, in particular, a vibration gyro having a bias correcting function, produced by a Micro-Electro-Mechanical-Systems (MEMS).
2. Description of the Related Art
Due to a micromachining technique rapidly developed since the 1990s, a large quantity of sensor structures can be produced in one process. For example, a bulk silicon wafer is bonded onto a silicon substrate having an insulating film or a glass substrate, and then the wafer is processed by chemical etching, such as wet etching or dry etching so as to form a mechanical sensor structure. As a sensor based on the MEMS technique, an acceleration sensor or a vibration gyro may be used, for example, in many fields including an automobile, inertia navigation, a digital camera, a game machine, etc.
In particular, a vibration gyro utilizes Coriolis force generated when a movable article capable of vibrating in one direction is subject to rotational motion. When a vibrating movable mass is subject to rotational motion, the movable mass is subject to Coriolis force which acts in the direction perpendicular to both the vibrating direction and the rotational direction, and then the movable mass is displaced in the acting direction of the Coriolis force. The movable mass is supported by a spring which allows the mass to be displaced in the acting direction, and thus the Coriolis force and an angular velocity generating the Coriolis force can be detected based on the displacement of the movable mass. The displacement of the movable mass can be determined based on, for example, capacitance change of a parallel plate-type capacitor or a comb-type capacitor, having a pair of parallel plate structures or a pair of comb structures, wherein one of the structures is fixed and the other is movable together with the movable mass.
As a means for improving output stability of a vibration gyro, a means for reducing or eliminating a leakage output (or a quadrature error) may be used. In the prior art documents below, a vibration gyro having a correcting means, which reduces the quadrature error for improving the bias stability, is disclosed.
In Japanese Unexamined Patent Publication (Kohyo) No. 2007-513344 (JPP'344), a Coriolis angular velocity meter having a first resonator 1, wherein first resonator 1 is configured as a combination system of first and second linear oscillators 3 and 4, first oscillator 3 is connected to a frame of the angular velocity meter via first spring elements 51 to 54, and second oscillator 4 is connected to first oscillator 3 via second spring elements 61 and 62. The angular velocity meter generates an electrostatic field (111′, 112′, 101 to 104) capable of alternately changing alignment of oscillators 3 and 4. The electrostatic field has a device which generates constant load for changing alignment angles of first spring elements 51 to 54 relative to frames 73 and 74 of the angular velocity meter and/or alignment angles of second spring elements 61 and 62 relative to first oscillator 3; a device (45, 47) for determining a quadrature (orthogonal) bias of the Coriolis angular velocity meter; and a control loop (55, 56, 57) which controls the intensity of the electrostatic field corresponding to the determined quadrature bias.
In the angular velocity meter described in JPP'344, the first resonator (or first vibrating body) is displaceably supported by the four bended elements (or first spring elements) in the X-direction (X1 direction) parallel to the substrate surface, and the second resonator (or second vibrating body) is displaceably supported, inside the first vibrating body, by the two bent elements (or second spring elements) in the Y-direction (X2 direction) parallel to the substrate surface and perpendicular to X1 direction. The first vibrating body is driven so that the displacement oscillation of the first vibrating body occurs, when the angular velocity about the Z-axis is input, the displacement oscillation of the second vibrating body in the Y-direction occurs due to Coriolis force. By detecting the displacement oscillation in the Y-direction, the angular velocity about the Z-axis may be determined. In JPP'344, as shown in FIG. 3, the structures, each including the first and second vibrating bodies, are positioned on right and left sides, and the first vibrating bodies are connected to each other by a connecting spring so that the first vibrating bodies are driven at the opposite phases. It can be understood that, by detecting the reverse phase displacement of the second vibrating body at the time when the angular velocity is input, a displacement output having relatively small error, proportional to the angular velocity, may be obtained.
In the angular velocity meter described in JPP'344, when the first oscillators are driven at the opposite phases in the X-direction, the internal second oscillators are vibrated in the Y-direction (i.e., the sensing direction of the Coriolis force) due to structural unbalance caused by manufacturing tolerances of each component of the velocity meter. This is because the orthogonality between the driving direction (X-direction) and the sensing direction (Y-direction) is not maintained. Therefore, in JPP'344, a correction structure having an excitation electrode is arranged near the center of the second oscillator, so that the orthogonality is maintained (i.e., the alignment in the vibration direction is adjusted). Concretely, a DC voltage for correction, corresponding to a bias output voltage obtained by a sensor signal processing circuit, is applied to the excitation electrode, whereby the alignment is adjusted by electrostatic force. However, in JPP'344, due to the driving vibration of the first oscillator, the second oscillator suspended within the vibrating first oscillator is also excited in the X-direction by the same amount as the first oscillator. Therefore, in the configuration having the manufacturing tolerances, a gap (or a capacitance) between the second oscillator and a sensing fixed electrode for sensing a Y-direction displacement is varied. Accordingly, the configuration of JPP'344 may include an instable bias factor which may cause an error to be corrected, even in an ideal condition wherein the orthogonality is maintained without generating an alignment error.
Further, JPP'344 discloses a method for compensating the orthogonal bias, concretely, a closed loop method for determining a DC voltage applied to the excitation electrode, with reference to an orthogonal bias component which is generated when the orthogonality is not maintained. However, when input angular velocity Ω is alternately changed (AC-like), the orthogonal bias is generated even when the orthogonality is maintained, in view of the principle of the vibration gyro. Therefore, the appropriate compensation may not be carried out by the closed loop method with reference to the orthogonal bias component.